The disclosure relates generally to gas cooled generators, and more particularly, to detectors for detecting, among other things, seal leakage or seal oil contamination in a gas cooled generator.
Electric generators having higher ratings are often cooled using pressurized gas, e.g., hydrogen, as the coolant. The casing of the generator contains the cooling gas, and the rotor shaft extends through the gas tight enclosure of the casing. The higher the hydrogen purity within the casing, typically, the more efficient the generator operates. Conversely, increased hydrogen consumption and low hydrogen purities within the generator present problems that may lead to forced outages. Two causes of high hydrogen consumption include static seal leakage from the casing and shaft seal leakage. The leakage rates in these areas are driven by assembly and the maintenance of the shaft sealing system. A third area of hydrogen consumption is the gas scavenge rate from the generator end cavities. This leakage rate is controlled by the machine operator and/or control system.
Leakage of hydrogen cooling gas from the static seals within the casing is a common occurrence over time. For example, potential areas of leakage in the casing may include: high voltage bushings, glands, collector terminals, end shield horizontal and vertical joints, access cover flange joints, and hydrogen seal casing vertical joints.
A shaft seal system is positioned in end housings coupled to the casing and seals against the rotor shaft to substantially prevent escape of the cooling gas. The shaft seal system may use a pair of seal rings that envelope the shaft circumferentially. The seal system may also contain a pair of labyrinth seals. Labyrinth seals are located generally inside of the seal rings, maintaining tight clearance with the shaft in an attempt to prevent oil and entrained air from exiting the seal area, a cavity about the seal rings and any labyrinth seals, and leaking through the end housing. Seal oil is made to pass through the clearance between the shaft and seal rings at a pressure higher than the cooling gas pressure in the casing. Anomalies with the shaft seal system can lead to significantly lower hydrogen purities than the expected levels or higher hydrogen consumption because the hydrogen escapes from the casing. In the seal area, seal oil gives up entrained air and dissolves the surrounding hydrogen due to the partial pressure change resulting in a decrease in the hydrogen purity levels. In order to maintain purity levels, continuous scavenging (i.e., replacement of a relatively low purity gas mixture with pure hydrogen) is performed. The normal scavenging levels, however, sometimes fail to meet purity requirements. At normal scavenging levels, the hydrogen consumption may be within acceptable limits, but leads to lower running purity. In many cases, the scavenging levels are increased to a higher scavenge rate setting resulting in higher hydrogen consumption in an attempt to raise the hydrogen purities to normal.
In addition to the above issues, over time, shaft seal ring clearance with the rotor may increase. In some instances, seal oil contamination can occur, which may damage the shaft seal rings. This situation may also result in an increased seal oil flow rate either due to increased seal clearance, which reduces the gas purity in the seal areas due to increased air carried by the seal oil. Subsequently, the casing purity may drop due to diffusion of this gas across the gap between the oil deflector and the rotor body.